1). Field of the Invention
This invention relates to phase change thermal interface materials (TIM's) comprising polyester resins and electronic devices with such TIM's.
2). Discussion of Related Art
Today's semiconductor devices, whether discrete power or logic integrated circuits, are smaller, run faster, do more and generate more heat. Due to increased power, some desktop microprocessors dissipate heat in the range of 50 to 100 watts. These power levels require thermal management techniques involving large-capacity heat sinks, good air flow, and careful management of thermal interface resistances. A well-designed thermal management program will keep operating temperatures within acceptable limits in order to optimize device performance and reliability.
Semiconductor devices are kept within their operating temperature limits by transferring junction-generated waste heat to the ambient environment, such as the surrounding room air. This is best accomplished by attaching a heat sink to the semiconductor package surface, thus increasing the heat transfer between the hot case and the cooling air. A heat sink is selected to provide optimum thermal performance. Once the correct heat sink has been selected, it must be carefully joined to the semiconductor package to ensure efficient heat transfer through this newly formed thermal interface.
Thermal materials have been used to join a semiconductor package and a heat sink, and to dissipate the heat from the semiconductor devices, such as microprocessors. A TIM typically comprises a polymer matrix and a thermally conductive filler. The TIM technologies used for electronic packages encompass several classes of materials such as epoxies, greases, gels, and phase change materials.
Metal filled epoxies commonly are highly conductive materials that thermally cure into highly crosslinked materials. However, metal filled epoxies exhibit localized phase separation due to package thermo-mechanical behavior and their high modulus leads to delamination at the interfaces.
Thermal greases display good wetting and ability to conform to the interfaces, no post-dispense processing, and high-bulk thermal conductivity. However, greases tend to migrate out from between the interfaces under cyclical stresses encountered during temperature cycling, a phenomenon known as “pump out.”
Gels typically comprise a crosslinkable silicone polymer, such as vinyl-terminated silicone polymer, a crosslinker, and a thermally conductive filler. After cure, gels are crosslinked filled polymers, and the crosslinking reaction provides cohesive strength to circumvent pump-out issues exhibited by greases during temperature cycling. Their modulus (E′) is low enough so that the material can still dissipate internal stresses and prevent interfacial delamination, but not low enough to survive the reliability-stressing test.
Phase change materials (PCMs) are in a class of materials that undergo a transition from a solid to a liquid phase with the application of heat. These materials are in a solid state at room temperature and are in a liquid state at die operating temperatures. When in the liquid state, PCMs readily conform to surfaces and provide low thermal interfacial resistance. PCMs offer ease of handling and processing due to their availability in a film form and the lack of post-dispense processing. However, from a formulation point, the polymer and filler combinations that have been utilized in PCMs restrict the bulk thermal conductivities of these materials. In general pump-out, bleed-out, and dry-out are continuing reliability issues for phase change TIMs. Commercial phase change thermal interface materials (such as Chomerics T454) are polyolefin-based. Because of their hydrocarbon chemical structure, polyolefins rapidly degrade in oxygen above about 125° C., with a corresponding increase in thermal resistance (Rja); see FIG. 1. This presents a high risk for thermo-oxidative degradation over time at operating temperatures below about 110° C. and is a show-stopper for applications with operating temperatures above 110° C.